Many experiments in physics, biology, chemistry and material science require short bursts of X-ray radiation to study the detailed time response of a sample under a particular excitation [1].

X-ray techniques are extremely important for the investigation of the chemistry and structure of materials. In a typical experiment, the system to be analysed is excited by a short X-ray burst and information about the response is obtained by studying the fluorescence emitted during the system’s de-excitation. In a pump-probe experiment, the sample is pumped in a metastable state by laser excitation and a delayed X-ray pulse is used to monitor the time evolution of the system.

The duration of the X-ray pulse is critical to a pump-probe experiment. Since the timescale involved can be smaller than a microsecond, X-ray pulses with duration less than a microsecond are required. Short-duration X-ray bursts are produced when a continuous X-ray beam passes through a mechanical shutter that is open for a short time. The major limitations of rotating mechanical shutters are their relatively large size and the constraints imposed by the operating environment. For example, they may have to work under vacuum conditions, especially if a very short X-ray pulse is required.

A combination of X-ray techniques and micro-electro-mechanical systems (MEMS) technology has recently been demonstrated in a setup where a modulated X-ray beam was used to excite a mechanical system around its first order resonance [2]. We show here that the inverse mechanism can be used to modulate an X-ray beam.

Figure 151 shows the experimental setup. The mechanical system is a standard Si (100) atomic force microscope (AFM) cantilever with dimensions 300 x 35 x 2 µm3. The cantilever displacement is measured by the interference between the light reflected from the end of a cleaved optic fibre and the beam reflected by the back of the lever. When at rest, the cantilever is in Bragg conditions and the photodiode will detect a constant photon flux. Whereas, when the AFM lever is excited by a piezo-electric ceramic, the X-ray incidence angle is modified by the lever motion. If the lever oscillation amplitude is larger than the Bragg peak width, the cantilever will periodically sweep through the Bragg conditions. Therefore, a periodically-modulated current will be measured at the output of the photodiode. The amplitude of the photon flux oscillation is thus directly controlled by the oscillation amplitude of the cantilever.

Fig. 151: Schematic representation of the experimental setup. The AFM cantilever (red) is used to modulate the X-ray beam (blue) impacting around the Bragg angle. The reflection of the X-ray beam is detected with a photodiode. The lever position is detected via the optical fibre (white).

The experiment was carried out at the surface X-ray diffraction (SXRD) beamline, ID03. The incoming X-ray beam set to 18.98 keV impinged on the cantilever at rest at Bragg condition and the X-ray photo detector was positioned at the corresponding 2 angle. The spot size of the incoming beam was 50 x 50 µm2 with 1010 ph/s. Figure 152a shows the cantilever oscillation amplitude and phase lag as a function of frequency when the mechanical system is mechanically excited at the resonance. Figure 152b shows the current (amplitude and phase) at the output of the X-ray photodiode as a function of the frequency.

Fig. 152: a) Optically measured response of the AFM cantilever when it is excited around its first resonant frequency. b) Diode photo-current measured by the photodiode at 2 Bragg position. In both a) and b) the black curve represents the oscillation amplitude and the red curve the phase lag.

We have shown that an X-ray beam can be modulated in intensity by an oscillating AFM lever. In this experiment the X-ray beam was chopped at 13 kHz. This operating frequency was related to the resonance frequency of the AFM lever used, however it could easily be increased to values much higher than 100 kHz by using others types of cantilever. Specific setups based on MEMS could be designed so that efficient X-ray choppers could operate at frequencies in the MHz regime, potentially opening a wealth of new experiments based on X-ray examination of time dependent processes requiring high repetition speed.



A. Siria (a,b), O. Dhez (c), W. Schwartz (a,b), G. Torricelli (d), F. Comin (c) and J. Chevrier (a).
(a) Institut Neel, CNRS-Université Joseph Fourier, Grenoble (France)
(b) CEA/LETI-MINATEC, Grenoble (France)
(c) ESRF
(d) Department of Physics and Astronomy, University of Leicester (UK)


[1] C. Rischel, et al., Nature 390, 490-492 (1997).
[2] A. Siria, et al., Nanotechnology 19, 445501 (2008).